1. Field of the Invention
This invention relates generally to information reproducing systems capable of accessing a desired data or information record track on a disc-shape information carrier, and more particularly, to track jump servo systems for disc players having servos for positioning a transducer means rapidly and accurately to a desired record track location on a disc surface.
2. Background of the Invention
Digital record discs for recording information signals such as digitized audio signals, video or image signals have been quite common as well as record disc reproducing apparatus for reproducing the recorded information data. Further, some digital record discs, e.g., so-called CDs (Compact Discs) have recently become used as ROM (Read Only Memory) devices for computers, called CD-ROMs. On such a digital record disc, digital data intended for high density recording are recorded in the form of a spiral bit string on one side thereof and reproduced through reading the bit string by means of transducer means such as an optical pick-up device using laser beams. The optical pick-up device of the record disc reproducing apparatus follows the bit string by means of a tracking control system. CD players or video disc players are further used to process the read-out signal in a predetermined manner so as to convert the signal into a replica of the original analog form signal which was recorded, i.e., an audio or video signal as an output.
As is well known, address information may also be recorded on the record disc which may be detected for search and track jump operations so that reproduction at a desired track location can be easily and quickly achieved. In particular, a selected address corresponding to the desired track location is preset, and the pick-up device is rapidly moved across or transverse to the track until the desired track corresponding to the selected address is reached. During such searching, a distance or a number of tracks that the pick-up device should jump or traverse is calculated by, for example, a computer in accordance with the address of the track at which the pick-up device is presently located and the address of the desired track. Then a fixed voltage signal is supplied to a pick-up drive motor to move the pick-up device radially across the disc surface. During the search operation, tracks that the pick-up device crosses are counted one by one. When the counted number of tracks jumped coincides with the desired number of tracks jumped, as calculated, the pick-up device is stopped and track servo control recommences. Tracks are counted, generally, by counting level changes or pulses in a tracking error signal which is responsive to whether the pick-up device is in or out of a track. In other words, the tracking error signal indicates deviations of the pick-up device from the center of the record track.
However, conventional track jump servo control systems are unable to increase the moving speed of the pick-up device over a predetermined value. As is well known, the information tracks are actually strings of pits to which the pick-up device can respond respectively arranged in a line. Obviously, the pits are separated from each other with spaces to which the pick-up device fails to respond. Radial movement of the pick-up device is carried out with the disc in revolution. Therefore, when the pick-up device is moved too fast radially relative to the disc surface, occasionally the pickup device will pass through a track without crossing any pit in the respective pit string. No pulse is produced in the tracking error signal, despite the pick-up device crossing a track, so that an error is introduced in the track count. Accordingly, this causes the pick-up device to be erroneously placed on a track different from the desired track.
In the past, the moving speed of the pickup device in the search operation had been limited to a speed which ensures that the pick-up device will cross a pit in every track. The maximum speed is approximately the speed at which a frequency of the level changes or the pulses of the tracking error signal is close to a frequency of pits during the revolution of the disc. Such a limitation of the moving speed of the pick-up device becomes an impediment for shortening the access time to a desired track location. Especially, the limitation is particularly serious in the recently developed CD-ROMs for which a shortening of the access time has been eagerly demanded.
In a short distance search operation of only a few tracks, the pick-up device can jump to the desired track relatively accurately. In a long distance search operation, however, to a track located dozens of tracks away from the track at which the pick-up device is presently located, the pick-up device does not become accurately placed on the desired track for a long time. During a long distance search operation, the actual distance that the pick-up device has moved is apt to differ from the desired distance calculated by the computer. As the distance the pick-up device should jump or traverse becomes longer, a track that the pick-up device has been placed at the end of the jump may differ more from the desired track due to mechanical loss or non-uniformity in the mechanism for transferring the pick-up device, inertia of the pick-up device, temperature conditions and the like.
Therefore, the search operation includes repeated track jumps so that the pick-up device gradually approaches the desired track location. After every track jump in the search operation, the track address is checked to determine whether a track where the pick-up device has been placed after the track jump coincides with the desired track or not.
As a result, it is difficult to ensure that the pick-up device jumps over or traverses rapidly and accurately the record tracks to a desired track location on the disc.